One of metabolome measurement methods or a capillary electrophoresis-mass spectrometry (CE-MS) method is very effective for measurements of ionic metabolites in various biological samples and has a very high specificity because most objects to be measured will not overlap with those by the gas chromatography-mass spectrometry (GC-MS) method or the liquid chromatography-mass spectrometry (LC-MS) method.
In general, in the CE-MS, as shown in FIG. 1, for example, at the top portion of a stainless needle 12 built in an electrospray interface (ESI) spray (hereafter simply referred to as a spray) 10 called the nebulizer, a buffer electrophoretic liquid (hereafter simply referred to as the electrophoretic liquid or buffer) 16 coming out of a capillary 14 is mixed with a solution containing an organic solvent called a sheath liquid 18. Then, from outside thereof, a nebulizer gas, for example, a nitrogen gas for accelerating ionization by creating fine droplets is sprayed, thereby applying voltages for electrophoresis and ionizing metabolites in the electrophoretic liquid 16 (see Patent Literatures 1 to 3). The sheath liquid 18 enables stable measurements.
However, a big problem with the current CE-MS method employing the sheath liquid 18 is that the concentration sensitivity (the detection sensitivity when samples having the same concentration are measured) is inferior as compared with another metabolome analysis technique.
The reason why the sensitivity is degraded in the CE-MS method is because the electrophoretic liquid 16 coming out of the capillary 14 is mixed with the sheath liquid 18 at the tip portion of the aforementioned nebulizer (10), thereby causing the object being measured in the sample to be diluted. For example, in the measurement conditions typically employed by the inventors, the dilution factor was calculated. It was found that the dilution factor was about 200 times. Because, the flow rate of the electrophoretic liquid 16 was 50 nL/min, whereas the flow rate of the sheath liquid 18 was 10 μL/min.
Thus, if the metabolite can be less diluted by the sheath liquid 18 at the tip portion of the nebulizer (10), or if a sheathless measurement method is possible without using the sheath liquid 18, it can be expected that the concentration sensitivity in the CE-MS is increased to a maximum of 200 times.
In the sheathless method, the sensitivity can be expected to increase due to no dilution at the outlet of the capillary 14, whereas it is difficult to perform CE-MS measurements with stability because there is no sheath liquid 18. The sheathless CE-MS methods reported so far can be largely divided into three as below.
(1) As shown in FIG. 2, a method for electrophoresis in which a minute hole is opened on the capillary 14 to directly embed and secure with an adhesive 24 an electrode 22 in the capillary 14 (Non-Patent Literature 1).
(2) As shown in FIG. 3, a method for electrophoresis in which an electrically conductive metal (for example, gold) 26 is evaporated onto the outlet of the capillary 14 (Non-Patent Literature 2).
(3) A method in which while typical electrophoresis is being performed with an electrophoretic liquid reservoir provided at some midpoint of a capillary, a compound in the electrophoretic liquid is migrated off-line with the help of an electroosmotic flow (a liquid flow that naturally occurs when a voltage is applied) EOF (Non-Patent Literature 3).